1. Technical Field
The present invention relates to optical communications and, more particularly, to systems and methods for compensating for chromatic dispersion in optical media.
2. Description of the Related Art
Chromatic dispersion (CD) manifests itself as the wavelength spreading of a pulse of light as it travels over great distances. Optical lasers output pulses of light with a finite spectrum. The longer the fiber over which the pulse travels, the more the pulse spreads out, such that an originally sharp wavelength boundary bleeds into neighboring wavelengths. Difficulties arise when the resulting energy from a pulse begins to interfere with that of an adjacent pulse. This is called inter-symbol interference in the electrical domain. The spreading of symbols across one another causes reception errors, where the receiver side of the link cannot easily distinguish between symbols because they are no longer at ideal levels. Depending on the fiber, pulse spreading may cross several unit intervals. A dispersion of one unit interval means that adjacent symbols within a given symbol will begin to interfere.
CD compensation has been performed optically, electrically, and with digital signal processing. For optical compensation, a dispersion compensating fiber is added in the transmission path. The fiber reverses dispersion, but attenuates the transmission line, such that the signal-to-noise ratio is reduced.
Electrical CD compensation reshapes a received electrical signal in the frequency domain and may use one of a variety of equalizing algorithms such as continuous-time filters, decision feedback equalizing, and maximum likelihood sequence estimation. Electrical CD compensation is limited, however, to compensating over a few unit intervals. As a result, electrical compensation is inadequate for long-distance transmissions.
Digital signal processing has a number of useful properties that overcome the challenges of optical and electrical CD compensation, but it needs considerably more processing power to accomplish. For example, finite impulse response filters used in digital signal processing often need hundreds of taps to effectively compensate for CD, which is not possible for currently available high-speed signal processing chips.